Heater cartridge resistance

A few months ago I purchased two 12v 40 W heater cartridges only to find when I measured up the resistance yesterday that one was 12 Ohms and the other 9 Ohms - both pretty much unusable. I also purchased two 24V 40 W cartridges from another supplier and they both measure up at 13.5 Ohms - pretty much on the nail of what I would expect.
What is the normal resistance of the 12V 40W heater? Any recommendations of where to get good ones with right resistance and no shorts to case?

Thanks,

Mike

Ohms law. Your 40W 12V heater should have 3.6 ohms. Your 24V 40W should be approx 14.4 ohms.

V x I=P

I x R=V

3.6 Ohms was also what I calculated - for the 24V I allowed within 10% as being "on the nail". What do the 12V heaters typically measure up to.
Where can I get some that are somewhere in the right ball park?

Mike

Edited 1 time(s). Last edit at 11/12/2013 06:46AM by leadinglights.